The invention relates in general to an electronic circuit designed for receiving a first supply voltage between a first supply connection terminal and a second supply connection terminal and for receiving a second supply voltage between the first supply connection terminal and a third supply connection terminal, comprising a DA converter for converting a digital signal with a voltage range which is at most equal to the first supply voltage into an analog signal with a voltage range which is at most equal to the second supply voltage, and more in particular to DA converters in integrated circuits or ICs.
Such electronic circuits are known from the general prior art. The general trend is towards designing ICs which operate at increasingly lower supply voltages. This has the result inter alia that the analog signal supplied by the DA converter has an ever smaller amplitude. This is because the peak-to-peak value of the analog signal cannot be greater than the value of the supply voltage. The lower maximum vale of the analog signal will also adversely affect the signal-to-noise ratio of the analog signal. Another disadvantage is that these modem ICs will have to deal with voltages higher than the maximum admissible operating voltages for modem ICs because they have to be able to communicate with ICs of older types, which operate at higher supply voltages. The latter problem is solved in the prior art in that the core of the DA converter in the modern IC is supplied from the comparatively low first supply voltage, and in that furthermore an amplifier is coupled behind the DA converter, which is connected to the comparatively high second supply voltage. The output of the amplifier then delivers an analog signal which ha a hither amplitude than the analog signal at the direct output of the DA converter. The amplifier must be designed such that it can cope with the higher second supply voltage. The signal-to-noise ratio mentioned above, however, is not improved thereby because the analog signal level at the direct output of the DA converter, i.e. at the input of the amplifier, is still limited by the value of the lower first supply voltage.
It is accordingly an object of the invention to provide an electronic circuit with a DA convener, which DA converter is capable of communicating with other electronic circuits which are supplied with a higher supply voltage, and which DA converter delivers an analog output signal with an improved signal-to-noise ratio.
According to the invention, the electronic circuit mentioned in the opening paragraph is for this purpose characterized in that the DA converter comprises: conversion resistors and coupling means for coupling a number of said conversion resistors between the first supply connection terminal and an output terminal of the DA converter, and for coupling the remaining number of conversion resistors between the third supply terminal and the output terminal, said number being dependent on the data content of the digital signal. In contrast to the prior art, the conversion resistors are coupled to the third supply connection terminal for receiving the second, comparatively high supply voltage, so that the analog signal at the output of the DA converter is no longer limited by the comparatively low first supply voltage. As a result, an analog signal with an enhanced amplitude level can be delivered at the direct output of the DA converter. The signal-to-noise ratio of the analog signal can thus be higher.
A further advantage of the electronic circuit according to the invention is that only the coupling means need be constructed such that they are resistant to the comparatively high second supply voltage, because resistors which can cope with a comparatively high voltage may be used as the conversion resistors. The resistors may be manufactured from, for example, polycrystalline silicon.
An embodiment of an electronic circuit according to the invention is characterized in that the coupling means comprise drivers which are coupled between the first supply connection terminal and the third supply connection terminal, and in that each conversion resistor is coupled by a first connection point to the output terminal and by a second connection point separately to an output of the associated separate driver.
Each separate driver is constructed such that it comprises no components across which a potential difference can arise higher than the maximum admissible IC process voltage, in spite of the fact that the separate driver is supplied from a comparatively high supply voltage which is higher than the process supply voltage of the IC process in which the separate driver is implemented.
Advantageous embodiments of said driver are defined in claims 3 to 10.
An embodiment of an electronic circuit is characterized in that the DA converter comprises a separate digital voltage level shifter for each driver, with a first supply connection point which is coupled to the first supply connection terminal, with a second supply connection point which is coupled to the second supply connection terminal, with a third supply connection point which is coupled to the third supply connection terminal, with a first output which is coupled to the input of the first buffer, with a second output which is coupled to the input of the second buffer, and with an input, and in that the DA converter in addition comprises synchronization means for synchronizing the data bits of the digital signal, separate inputs of the synchronization means being coupled so as to receive the separate data bits, while the synchronization means have a clock input for receiving a clock signal, and the synchronization means have separate outputs which are coupled to the separate inputs of the separate digital voltage level shifters.
The separate voltage shifters in this embodiment ensure that the first buffers and the second buffers are controlled with signals having the desired voltage levels. The synchronization means ensure that the data bits are read substantially simultaneously under the control of the clock signal.
An embodiment of an electronic circuit according to the invention is characterized in that the DA converter comprises a separate digital voltage level shifter for each driver, with a first supply connection point which is coupled to the first supply connection terminal, with a second supply connection point which is coupled to the second supply connection terminal, with a third supply connection point which is coupled to the third supply connection terminal, with a first output, with a second output, and with an input, the separate inputs of the voltage level shifters being coupled so as to receive the separate data bits of the digital signal, and in that the DA converter in addition comprises synchronization means for synchronizing the data bits, separate inputs of the synchronization means being coupled to the separate first outputs and the separate second outputs of the voltage level shifters, which synchronization means have a clock input for receiving a clock signal, and which synchronization means have separate outputs which are coupled to the separate first inputs of the first buffers and the separate second inputs of the second buffers. This is an alternative version of the preceding embodiment. The synchronization means are now situated not upstream of the level shifters, but in between the level shifters and the buffers.
Advantageous embodiments of the voltage level shifters mentioned above are defined in claims 13 to 16.
An embodiment of an electronic circuit according to the invention is characterized in that the electronic circuit comprises a current compensation circuit which is connected between the first supply connection terminal and the third supply connection terminal for receiving the second supply voltage, which current compensation circuit in the operational state is controlled from the digital signal in a manner such that the sum of the current consumption of the DA converter and the current consumption of the current compensation circuit is substantially independent of the data content of the digital signal. The current drawn by the DA converter is dependent on the data content of the digital signal. A certain wiring impedance is always present in series with the supply lines of the DA converter (ohmic resistance and self-inductance). Owing to the presence of this wiring impedance and owing to the fact that the power consumed by the DA converter is dependent on the data content of the digital signal, the DA converter is supplied with a voltage which contains a data-dependent component. Since the so-called Power Supply Rejection Ratio of the DA converter is comparatively low, a distortion of signals will arise in the DA converter in dependence on the value of this Power Supply Rejection Ratio. An undesirable signal crosstalk to other components of the electronic circuit may also arise.
Both the DA converter and the current compensation circuit have a data-dependent current consumption. The data-dependent component of the current consumption of the current compensation circuit has the same value as the data-dependent component of the current consumption of the DA converter. The two said current components, however, are mutually in counterphase. Since the supply connection points of the DA converter and of the current compensation circuit are connected to one another by means of very short wiring portions, substantially no data-dependent current will flow through said wiring impedances. This is because the data-dependent component of the DA converter and the data-dependent component of the current compensation circuit compensate one another. The result of this is that the DA converter is supplied with a supply voltage which is substantially independent of the data content of the digital signal. Signal distortion and signal crosstalk are thus avoided, even at a low Power Supply Rejection Ratio.
Advantageous embodiments of current compensation circuits according to the invention are specified in claims 19 to 22.